96 Mr. Brookes on a new Genus of the Order' tlodentia. 



author, indeed, appears to have doubted the propriety of this 

 location, and mentions with evident regret, that the loss of the 

 remains of the animal had prevented our becoming acquainted 

 with its organization, and ascertaining precisely its characters. 

 Fortunately, however, the animal, although obscured from notice 

 during so long a period, is yet in a condition for accurate and 

 minute examination. 



The description of its preserved skin and skeleton I have now 

 the honour of presenting to the notice of the Society ; and from 

 the structure of the latter especially, it will be evident that it 

 must be referred to a new genus, to which I propose to give the 

 name of Lagostomus. The form of the teeth, on which so much 

 stress is justly laid in characterizing genera, differs essentially 

 from that exhibited by all the other Rodentia ; from which it is 

 also distinguished not only by the number of its toes, but by 

 various other particulars of its osteology, which I shall now 

 proceed rapidly to describe, assuming occasionally as a point of 

 comparison the skeleton of the Dipus Sagitta, with which it has 

 been generically confounded. 



The upper surface of the cranium in Lagostomus exhibits the 

 usual form of that of the Rodentia, its sides being nearly pa- 

 rallel, and its occipital breadth scarcely exceeding its breadth 

 immediately anterior to the orbits. In Dipus, on the con- 

 trary, the outline is decidedly triangular, arising from the very 

 considerable dilatation of its hinder part, occasioned by the 

 extraordinary development of the mastoid processes of the tem- 

 poral bones, which are extremely delicate, and possess, as in 

 man, a cellular structure*. ' The 



• The Egyptian Jerboas being known to domiciliate themselves under bushes fre- 

 quented by the Cerastes, so that it frequently, or perhaps generally occurs, that where 

 the one, there the other is also found ; this particular osseous extension may be destined 

 by Nature, to give increased sensitiveness to the auditory organ, for the greater secuiity 

 pf the animal. In the Chlamyphorus truncatus there are two somewhat similar osseous 



tumours 



